Method of forming channel in thin film transistor using non-ionic excited species

ABSTRACT

In the production of channel etch type bottom gate thin film transistors, etching damage in a channel etch step is prevented to improve the transistor performance. The channel etch is performed using non-ionic excited species, such as hydrogen radicals and fluorine radicals, generated by contact-decomposition reaction which utilizes a metal heated by electric resistance heating. Alternatively, in place of the channel etch, a portion of the source/drain semiconductor thin film immediately above the channel is nitrided by a non-ionic nitrogen-containing decomposition product that is produced by contacting molecules of a chemical substance containing nitrogen atoms with a metal heated by electric resistance heating to decompose the chemical molecules.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to bottom gate thin filmtransistors (TFTs) for use in liquid crystal display (LCD) devices andorganic electroluminescent display devices. The invention also relatesto methods of producing the TFTs.

[0003] 2. Description of the Prior Art

[0004] Hydrogenated amorphous silicon (a—Si:H) TFTs are widely utilizedfor switching elements in liquid crystal display devices since they canbe accurately fabricated on inexpensive glass substrates by a lowtemperature process.

[0005] There are two types of structures for a—Si TFTs, a top gatestructure and a bottom gate structure. The top gate structure has adrawback that the interface between an a—Si:H thin film, which is laterformed into the channel, and a gate insulating film is oftencontaminated during the fabrication. In contrast, the bottom gatestructure is advantageous in that, since the a—Si:H thin film and thegate insulating film are successively produced without being exposed toatmosphere, the performance of the TFTs is not degraded by thecontamination and the electron mobility is larger than the top gateTFTs. Thus, for switching elements in liquid crystal display devices orthe like, bottom gate TFTs are advantageous. Bottom gate TFTs have twomajor types, etch stopped type and channel etch type (also referred toas back channel etch type). Channel etch type TFTs require lessphotomasks in the fabrication process than etch stopped type TFTs. Thismakes channel etch type bottom gate TFTs advantageous in terms ofmanufacturing cost, and therefore, channel etch type bottom gate TFTshave increasingly been favored recently.

[0006]FIGS. 7A to 7F show the production process of a channel etch typebottom gate TFT, each of the cross sectional views illustrating a stepof the production process. With reference to these figures, afabrication procedure of a channel etch type bottom gate TFT isdescribed below.

[0007] A gate electrode 62 is deposited on an insulating substrate 61 toa thickness of 200 nm by sputtering and thereafter patterned into anisland by photolithography and etching (FIG. 7A). Typically, the gateelectrode 62 is made of an aluminum film, or a layered film made of analuminum film and a film of a metal having a high melting point, such astitanium.

[0008] Subsequently, an SiN_(x)film, which serves as a gate insulatingfilm 63, is deposited to a thickness of 300 nm by plasma enhancedchemical vapor deposition (PECVD), and thereafter, without exposing thesurface to atmosphere, an a—Si:H film, which serves as a highresistivity semiconductor film 64, is deposited to a thickness of 200 nmby PECVD. Then, an n+ a—Si:H film, which serves as a low resistivitysemiconductor film 65, is deposited to a thickness of 20 nm by PECVD.Thereafter, stacked layers of the high resistivity semiconductor film 64and the low resistivity semiconductor film 65 are processed into anisland by photolithography and etching (FIG. 7B).

[0009] Subsequently, a source/drain electrode metal 66 is deposited bysputtering (FIG. 7C). Thereafter, a resist 67 is coated (FIG. 7D), andan opening is formed by photolithography and etching in a portionthereof which is located above the channel region. Thereafter, the lowresistivity semiconductor film 65 is etched with the use of the sameresist pattern to form a back channel (FIG. 7E).

[0010] This step is generally referred to as a channel etch step.

[0011] Subsequently, in order to protect the back channel exposed by theetching, a silicon nitride film serving as a passivation film 68 isdeposited to a thickness of 300 nm by CVD. Finally, an opening 69 forconnecting a pixel electrode is opened in a predetermined position inthe passivation film 68 by photolithography and etching. Thus, a TFT iscompleted.

SUMMARY OF THE INVENTION

[0012] However, the foregoing prior art method has at least thefollowing problems. In the prior art method, due to the fact that theetching selective ratio between the low resistivity semiconductor film65 and the high resistivity semiconductor film 64 is small, overetch iscaused in the channel etch step and thereby the high resistivitysemiconductor film 64 is considerably etched in addition to the lowresistivity semiconductor film 65 (FIG. 7E). When such overetch occurs,hydrogen in the back channel, which is composed of an a—Si:H film, islost, and etching damage is caused by which the film uniformity withrespect to the vertical orientation of the film is degraded. The etchingdamage deteriorates various TFT characteristics. For example, the fieldeffect mobility of the TFT decreases to about half.

[0013] To reduce the etching damage to the back channel, if the filmthickness of the high resistivity semiconductor film 64 is increased,the time required for the film deposition correspondingly increases,reducing efficiency in production. On the other hand, if the depositionrate is increased and the deposition time is thereby reduced, filmquality is degraded. In other words, both of the approaches haveproblems; the former increases fabrication cost due to an increase inproduction tact time, whereas the latter degrades production yields andTFT characteristics. Therefore, the prior art method cannot achieveefficient production of bottom gate TFTs which sufficiently function inhigh resolution display devices in which moving pictures are displayed.

[0014] Accordingly, it is a first object of the present invention toprovide a method of producing a bottom gate TFT that solves theforegoing and other problems in the prior art. It is a second object ofthe invention to provide a liquid crystal display device and an organicelectroluminescent display device to which the manufacturing method ofthe invention is applied. These and other objects are accomplished, inaccordance with the present invention, by the following embodimentswhich include a range of aspects.

[0015] Embodiment 1

[0016] According to a first aspect of the invention, there is provided amethod of producing a bottom gate thin film transistor, comprising thesteps of:

[0017] forming a gate electrode on an insulating substrate;

[0018] forming a gate insulating film over the gate electrode;

[0019] forming a first semiconductor thin film for a channel over thegate insulating film;

[0020] forming a second semiconductor thin film for a source and a drainover the first semiconductor thin film;

[0021] processing stacked layers of the first semiconductor thin filmand the second semiconductor thin film so as to be formed into anisland;

[0022] subsequent to the step of processing stacked layers, depositing asource/drain electrode metal over the stacked layers of the firstsemiconductor thin film and the second semiconductor thin film;

[0023] etching a region of the deposited source/drain electrode metal,the region being located above the channel, in the depth direction toexpose the second semiconductor thin film, whereby a source electrodeand a drain electrode are formed; and

[0024] etching away the exposed portion of the second semiconductor thinfilm in the depth direction with the use of a non-ionic excited speciesto form a channel.

[0025] In this fabrication method, non-ionic excited species are used inthe etching of the second semiconductor thin film for the source anddrain (so-called channel etch). The use of non-ionic excited speciesreduces etching damage to the back channel because the excited speciesare not accelerated by electric field. Therefore, the production yieldincreases, and reliability in product quality of the produced channeletch bottom gate TFTs remarkably improves.

[0026] According to a second aspect of the invention, the fabricationmethod of the first aspect may be such that the non-ionic excitedspecies is generated by bringing molecules of a chemical substance intocontact with a metal heated by electric resistance heating to decomposethe molecules of the chemical substance.

[0027] This fabrication method utilizes a catalytic CVD technique andmakes it possible to produce a large amount of non-ionic excited specieswith simple equipment. In addition, in this method(contact-decomposition reaction method), few ionic excited species aregenerated. It is noted that the molecules of the chemical substrateherein are meant to include molecules composed of a single element, suchas H₂ molecules.

[0028] According to a third aspect of the invention, the method of thesecond aspect may be such that the non-ionic excited species is aradical.

[0029] In non-ionic excited species, radicals have large energy.Accordingly, good etching efficiency can be obtained. Therefore, the useof radicals is preferable.

[0030] According to a fourth aspect of the invention, the method of thethird aspect may be such that the metal is selected from the groupconsisting of tungsten, tantalum, molybdenum, vanadium, platinum, andthorium, or is an alloy comprising at least two metals selected from thegroup consisting of tungsten, tantalum, molybdenum, vanadium, platinum,and thorium.

[0031] These metals function as catalysts in the contact-decompositionreaction. These metals are preferable because they have high meltingpoints and can be heated by electric resistance heating. Of thesemetals, tungsten is particularly preferable. This is because tungstenhas the highest melting point among all the metals, and thereforehydrogen gas can be very efficiently decomposed into radicals. Further,even if evaporated tungsten contaminates the silicon semiconductor, thecharacteristics of the TFTs are not degraded seriously insofar as theamount of the contaminant is little.

[0032] According to a fifth aspect of the invention, the method of thefourth aspect may further comprise, subsequent to the step of etchingaway the exposed portion of the second semiconductor thin film, forminga passivation film comprising a silicon nitride film in such a mannerthat the etched surface is not exposed to atmosphere.

[0033] When the surface of the channel is exposed to atmosphere, thesurface contamination occurs, which causes the degradation of TFTcharacteristics. For this reason, it is preferable that the surface ofthe channel be protected by a passivation film so as not to be exposedto atmosphere. In the present invention, the channel etch may beperformed by using a contact-decomposition reaction apparatus that canalso be utilized for the formation of the passivation film. Thereby, thepassivation film can be successively formed subsequent to the channeletch, and therefore the above-described method can be easily realized.

[0034] According to a sixth aspect of the invention, the method of thefifth aspect may be such that the first semiconductor thin film is athin film comprising silicon; and the second semiconductor thin film isa thin film comprising silicon and an n-type impurity.

[0035] When the channel etch using an excited species is employed for asemiconductor employing a silicon thin film or a silicon thin film whichcontains an n-type impurity, the advantageous effects achievable by thepresent invention are more saliently exhibited. It is to be noted,however, that the present invention is not limited to the use of thesesemiconductor films. For example, a silicon-germanium thin film may alsobe used.

[0036] According to a seventh aspect of the invention, the method of thesixth aspect may be such that the silicon comprises amorphous silicon orpolycrystalline silicon.

[0037] According to an eighth aspect of the invention, the method of thefirst aspect may be such that the non-ionic excited species is anon-ionic radical.

[0038] According to a ninth aspect of the invention, the method of thefirst aspect may further comprise, subsequent to the step of etchingaway the exposed portion of the second semiconductor thin film, forminga passivation film composed of a silicon nitride film in such a mannerthat the etched surface is not exposed to atmosphere.

[0039] According to a tenth aspect of the invention, the method of thesecond aspect may be such that the metal is selected from the groupconsisting of tungsten, tantalum, molybdenum, vanadium, platinum, andthorium, or is an alloy comprising at least two metals selected from thegroup consisting of tungsten, tantalum, molybdenum, vanadium, platinum,and thorium.

[0040] According to an 11th aspect of the invention, the method of thethird aspect may be such that the molecules of the chemical substancecomprise hydrogen, ammonia, or a mixture thereof.

[0041] These substances are easily decomposed and produce radicals whenthey make contact with a metal heated by electric resistance heating.Therefore, these substances are advantageous to efficiently performchannel etch.

[0042] According to a 12th aspect of the invention, the method of thethird aspect may be such that the non-ionic radical is a hydrogenradical.

[0043] Hydrogen radicals do not etch such metals as aluminum (Al) andtitanium (Ti) and therefore, the source/drain electrode metal made ofAl, Ti, or the like can be utilized as a mask in the channel etch. Forthis reason, the above-described method is advantageous for increasingproductivity in TFT fabrication. It is noted here that a technique ofutilizing the source/drain electrode metal as a mask is disclosed inJapanese Examined Patent Publication No. 6-30397. This technique employsCH₄ or NF₃ for the etching gas and Cr for the source/drain electrodemetal. The reason for the use of Cr is that, when a metal other than Cris used for the source/drain electrode, the source/drain electrode isdamaged by CH₄ or NF₃. The above-described method, on the other hand,employs hydrogen radicals, and therefore Ti and Al are usable for thesource/drain electrode metal.

[0044] According to a 13th aspect of the invention, the method of thethird aspect may be such that the non-ionic radical is a halogenradical.

[0045] The use of halogen radical is preferable in that, even when anative oxide film is present on the surface of the silicon thin film,which is a semiconductor thin film, halogen radicals are capable ofeasily etching the native oxide film. Therefore, the etching uniformityfor silicon does not degrade.

[0046] According to a 14th aspect of the invention, the method of the13th aspect may be such that the non-ionic halogen radical is a fluorineradical.

[0047] Fluorine radicals are desirable in that, for example, a desirableselective ratio is relatively easily obtained between metals, such asthe ones used for the source/drain metal, and silicon, which forms thesemiconductor thin film.

[0048] According to a 15th aspect of the invention, the method of thefirst aspect may be such that the step of etching is such that theexcited species is generated in a microwave plasma generating chamberprovided in isolation from an etching chamber in which the etching isperformed, and from the generated excited species, only non-ionicexcited species are selected and introduced into the etching chamber.

[0049] The excited species that are generated by a plasma generatingapparatus contain ionic excited species, and the ionic excited speciesare accelerated by a direct current electric field component generatedin the plasma generating apparatus. Therefore, when a plasma thatcontains ionic excited species is used for the channel etch, etchingdamage is caused to the channel. In the above-described method, however,only non-ionic excited species are selected to be used in the channeletch, and accordingly, the excited species are not accelerated by thedirect current electric field component. Thus, etching damage isreduced.

[0050] According to a 16th aspect of the invention, the method of the15th aspect may be such that the selected non-ionic excited species is anon-ionic radical.

[0051] According to a 17th aspect of the invention, the method of the15th aspect may further comprise, subsequent to the step of etching awaythe exposed portion of the second semiconductor thin film, forming apassivation film comprising a silicon nitride film in such a manner thatthe etched surface is not exposed to atmosphere.

[0052] When the etched surface is exposed to atmosphere, thecharacteristics of the transistors are degraded by the contamination ofthe exposed surface. The above-described method prevents the surfacefrom contamination, thereby improving production yields and reliabilityin product quality.

[0053] Embodiment 2

[0054] According to an 18th aspect of the invention, there is provided amethod of producing a bottom gate thin film transistor, comprising thesteps of:

[0055] forming a gate electrode on an insulating substrate;

[0056] forming a gate insulating film over the gate electrode;

[0057] forming a first semiconductor thin film for a channel over thegate insulating film;

[0058] forming a second semiconductor thin film for a source and a drainover the first semiconductor thin film;

[0059] processing stacked layers of the first semiconductor thin filmand the second semiconductor thin film into an island;

[0060] subsequent to the step of processing stacked layers, depositing asource/drain electrode metal over the stacked layers;

[0061] etching a region of the deposited source/drain electrode metal,the region being located above the channel, in the depth direction toexpose the second semiconductor thin film, whereby a source electrodeand a drain electrode are formed; and

[0062] nitriding the exposed portion of the second semiconductor thinfilm using a non-ionic nitrogen-containing decomposition product that isproduced by decomposing molecules of a chemical substance containingnitrogen atoms.

[0063] In the above-described method, a portion (nitrided region) of thesecond semiconductor thin film that is overlaid immediately above thechannel is nitrided using a non-ionic nitrogen-containing decompositionproduct that is produced by decomposing the molecules of a chemicalsubstance containing nitrogen atoms. In the nitriding step of theabove-described method, unnecessary nitriding of the first semiconductorthin film for the channel, which lies below the second semiconductorthin film, is avoided, and only the region to be nitrided in the secondsemiconductor film is: accurately nitrided. Therefore, good transistorcharacteristics are achieved. Moreover, the channel, which is formed ina layer below the nitrided region at the same time as the formation ofthe nitrided region, is protected by the nitrided film which is layeredthereover without being exposed to atmosphere. Thus, the above-describedmethod remarkably improves reliability and stability of TFTs.

[0064] Japanese Patent No. 3191745 discloses a method in which channeloxidizing or channel nitriding is used in place of a channel etch stepof the bottom gate TFTs. More specifically, this publication discloses amethod in which an amorphous silicon film made into an n-type is exposedin a plasma containing one of a) oxygen ions, b) both oxygen ions andnitrogen ions, and c) nitrogen radicals, to modify the n-type amorphoussilicon film into an insulating film of an oxynitrided film. However, itis known that in cases where the active layer is made of a—Si:H, the useof oxidizing degrades the transistor characteristics below the thresholdvoltage. Therefore, nitriding is advantageous over oxidizing.Nevertheless, the nitriding technique described in the foregoingpublication uses a plasma, and for this reason, this technique cannotobtain sufficient rate of nitriding. Moreover, a plasma contains a ionicdecomposition product. The ionic decomposition product is accelerated byelectric field and collides with the semiconductor film. Thereby, thesilicon film placed underneath the n-type amorphous silicon film isdamaged, and consequently, the transistor characteristics are degraded.In contrast, the above-described method of the present invention uses anon-ionic nitrogen-containing decomposition product, and therefore,damage to the back channel is little. Hence, the above-described methodachieves stable transistor characteristics.

[0065] According to a 19th aspect of the invention, the method of the18th aspect may be such that the non-ionic nitrogen-containingdecomposition product is generated by bringing a metal heated byelectric resistance heating into contact with the molecules of thechemical substance containing nitrogen atoms.

[0066] This method achieves efficient generation of the non-ionicnitrogen-containing decomposition product and thereby improvesproductivity.

[0067] According to a 20th aspect of the invention, the method of the19th aspect may be such that the molecules of the chemical substancecomprise ammonia.

[0068] When the molecules of the chemical substance comprise ammonia,the non-ionic nitrogen-containing decomposition product is efficientlygenerated by contact-decomposition reaction with the metal heated byelectric resistance heating. As a result, the nitriding proceedsswiftly.

[0069] According to a 21st aspect of the invention, the method of the19th aspect may be such that the metal is selected from the groupconsisting of tungsten, tantalum, molybdenum, vanadium, platinum, andthorium, or is an alloy comprising at least two metals selected from thegroup consisting of tungsten, tantalum, molybdenum, vanadium, platinum,and thorium.

[0070] These metals function as an excellent catalyst for thecontact-decomposition reaction. These metals are preferable because theyhave high melting points and can be heated by electric resistanceheating. Of these metals, tungsten is particularly preferable. This isbecause tungsten has the highest melting point among all the metals, andtherefore, the molecules of the chemical substance containing nitrogenatoms are very efficiently decomposed into radicals. Further, even ifevaporated tungsten contaminates the silicon semiconductor, thecharacteristics of the TFTs are not degraded seriously insofar as theamount of the contaminant is little.

[0071] According to a 22nd aspect of the invention, the method of the19th aspect may be such that the first semiconductor thin film is a thinfilm comprising silicon; and the second semiconductor thin film is athin film comprising silicon and an n-type impurity.

[0072] It is preferable that the channel nitriding using the non-ionicnitrogen-containing decomposition product is applied to a semiconductorfilm comprising silicon, because the advantageous effects achievable bythe present invention are thereby more saliently exhibited.

[0073] According to a 23rd aspect of the invention, the method of the22nd aspect may be such that the silicon comprises amorphous silicon orpolycrystalline silicon.

[0074] The channel nitriding using the non-ionic nitrogen-containingdecomposition product exhibits more salient advantageous effects whenapplied to a silicon thin film comprising amorphous silicon orpolycrystalline silicon.

[0075] Embodiment 3

[0076] According to a 24th aspect of the invention, there is provided abottom gate thin film transistor comprising:

[0077] a gate electrode formed on an insulating substrate;

[0078] a gate insulating film formed over the gate electrode;

[0079] a channel region comprising a first semiconductor thin filmstacked over the gate insulating film;

[0080] a source region and a drain region each comprising a secondsemiconductor thin film that is stacked over a region of the firstsemiconductor thin film exclusive of the channel region;

[0081] a source electrode and a drain electrode formed on the secondsemiconductor thin film; and

[0082] a passivation film composed of a silicon nitride film formed onthe channel;

[0083] wherein a portion of the channel contains at least one elementselected from the group consisting of tungsten, tantalum, molybdenum,vanadium, platinum, and thorium, the portion of the channel beingadjacent to a surface thereof which is in contact with the siliconnitride film, and the total atomic density of the at least one elementis in the range of from 1×10¹⁶·cm⁻³ to 1×10¹⁹·cm⁻³.

[0084] The present inventors have found that when non-ionic excitedspecies such as radicals are generated, the metal heated by electricresistance heating is evaporated and the evaporated metal, though in atrace amount, contaminates the first semiconductor thin film, whichforms the channel. However, the present inventors have also found thatwhen the metal is at least a metal selected from the group of tungsten,tantalum, molybdenum, vanadium, platinum, and thorium, adverse effectsof the contaminant on transistor characteristics are negligible insofaras the amount of the contaminant is an atomic density of from1×10¹⁶·cm⁻³ to 1×10¹⁹·cm⁻³. Based on these findings, the above-describedmethod of the invention was thus accomplished.

[0085] According to a 25th aspect of the invention, the bottom gate thinfilm transistor of the 22nd aspect may be such that the firstsemiconductor thin film is a thin film comprising silicon; and thesecond semiconductor thin film is a thin film comprising silicon and ann-type impurity.

[0086] According to a 26th aspect of the invention, the bottom gate thinfilm transistor of the 25th aspect may be such that the siliconcomprises amorphous silicon or polycrystalline silicon.

[0087] Embodiment 4

[0088] According to a 27th aspect of the invention, there is provided abottom gate thin film transistor comprising:

[0089] a gate electrode formed on an insulating substrate;

[0090] a gate insulating film formed over the gate electrode;

[0091] a channel formed of a first semiconductor thin film stacked overthe gate insulating film;

[0092] a source and a drain each formed of a second semiconductor thinfilm stacked over the first semiconductor thin film;

[0093] a nitrided region in which a portion of the second semiconductorthin film disposed immediately above the channel is nitrided; and

[0094] a source electrode and a drain electrode, each formed on aportion of the second semiconductor thin film exclusive of the nitridedregion;

[0095] wherein a portion of the channel contains at least one elementselected from the group consisting of tungsten, tantalum, molybdenum,vanadium, platinum, and thorium, the portion of the channel beingadjacent to a surface thereof which faces the nitrided region; and thetotal atomic density of the at least one element is in the range of from1×10¹⁶·cm⁻³ to 1×10¹⁹·cm⁻³.

[0096] According to a 28th aspect of the invention, the bottom gate thinfilm transistor of the 27th aspect may be such that the firstsemiconductor thin film is a thin film comprising silicon; and thesecond semiconductor thin film is a thin film comprising silicon and ann-type impurity.

[0097] According to a 29th aspect of the invention, the bottom gate thinfilm transistor of the 28th aspect may be such that the siliconcomprises amorphous silicon or polycrystalline silicon.

[0098] Embodiment 5

[0099] According to a 30th aspect of the invention, there is provided aliquid crystal display device comprising:

[0100] a first substrate comprising a plurality of scan electrodes, aplurality of data electrodes intersecting the scan electrodes, aplurality of thin film transistors provided at the intersectionalpositions of the scan electrodes and the data electrodes so that atleast one of the plurality of thin film transistors is provided at eachof the intersectional positions, and a plurality of pixel electrodesconnected to the thin film transistors;

[0101] a second substrate comprising a counter electrode opposed to thepixel electrodes; and

[0102] a liquid crystal sandwiched between the first substrate and thesecond substrate;

[0103] wherein each of the thin film transistors is a bottom gate thinfilm transistor according to any one of the foregoing 24th to 29thaspects of the invention.

[0104] Any of the bottom gate thin film transistors described in theabove 24th to 29th aspects of the invention sufficiently exhibit thecharacteristics when used for the TFTs in a liquid crystal displaydevice. Therefore, the above-described configuration achieves a liquidcrystal display device that is excellent in terms of stable operation.

[0105] Embodiment 6

[0106] According to a 31 aspect of the invention, there is provided anorganic electroluminescent display device comprising:

[0107] a first substrate comprising a plurality of scan electrodes, aplurality of data electrodes intersecting the scan electrodes, aplurality of thin film transistors provided at the intersectionalpositions of the scan electrodes and the data electrodes so that atleast one of the thin film transistors is provided at each of theintersectional positions, and a plurality of pixel electrodes connectedto the thin film transistors;

[0108] a second substrate comprising a counter electrode opposed to thepixel electrodes; and

[0109] a layer comprising an organic electroluminescent material, thelayer being sandwiched between the first substrate and the secondsubstrate;

[0110] wherein each of the thin film transistors is a bottom gate thinfilm transistor according to any one of the foregoing 24th to 29thaspects of the invention.

[0111] Any of the bottom gate thin film transistors described in theabove 24th to 29th aspects of the invention sufficiently exhibit thecharacteristics when used for switching elements in an organicelectroluminescent display device Therefore, the above-describedconfiguration achieves an organic electroluminescent display device thatis excellent in terms of stable operation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0112] For a more complete understanding of the present invention, andthe advantages thereof, reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings, inwhich;

[0113]FIGS. 1A to 1G are schematic cross sectional views showing thesteps of a producing method of a bottom gate TFT according to Example 1.

[0114]FIG. 2 is a schematic view showing an etching and nitridingapparatus used in Examples 1 and 2.

[0115]FIG. 3 is a schematic plan view showing the bottom gate TFTaccording to Example 1.

[0116]FIGS. 4A to 4G are schematic cross sectional views showing thesteps of a producing method of a bottom gate TFT according to Example 2.

[0117]FIG. 5 is a schematic view showing another example of an etchingand nitriding apparatus usable for Examples 1 and 2.

[0118]FIG. 6 is a perspective view showing the general construction ofdisplay devices of Examples 3 to 5; and

[0119]FIGS. 7A to 7F are schematic cross sectional views showing thesteps of a prior art producing method of a bottom gate TFT.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0120] Now, preferred embodiments of bottom gate TFTs and methods ofproducing the TFTs are discussed with reference to specific examples. Inthe examples below, a—Si:H is used for the high resistivitysemiconductor film, and n-type a—Si:H doped with phosphorus is used forthe low resistivity semiconductor film. It is to be understood, however,that the description is for illustrative purposes only. Thesemiconductor films may be formed of, for example, polycrystallinesilicon (including microcrystalline silicon), amorphous silicongermanium, polycrystalline silicon germanium. It is also noted that thehigh resistivity semiconductor film is also referred to herein as afirst semiconductor thin film or a semiconductor thin film for achannel, and the low resistivity semiconductor film is also referred toherein as a second semiconductor thin film or a semiconductor thin filmfor a source/drain.

EXAMPLE 1

[0121] Example 1 pertains to a method of producing a bottom gate TFT,the method employing a channel etch step utilizing hydrogen radicals.Referring to FIGS. 1A to 1G, the details of the method of Example 1 arepresented below.

[0122] First, a gate electrode metal is deposited on an insulatingsubstrate to a thickness of 200 nm by sputtering and is then processedinto an island by photolithography and etching to form a gate electrode2 (FIG. 1A). The gate electrode metal may be, for example, aluminum or alayered film composed of aluminum and a metal having a high meltingpoint, such as titanium.

[0123] Subsequently, an SiN_(x) film, which serves as a gate insulatingfilm 3, is deposited to a thickness of 300 nm by CVD. Thereafter, ana—Si:H film, which serves as a high resistivity semiconductor film 4 andlater forms a channel layer, is deposited to a thickness of 200 nm byCVD. Thereafter, an n+ a—Si:H film, which serves as a low resistivitysemiconductor film 5, is deposited to a thickness of 20 nm by CVD. Then,by photolithography and etching, the stacked layers of the highresistivity semiconductor film 4 and the low resistivity semiconductorfilm 5 are processed into an island (FIG. 1B).

[0124] Subsequently, a source/drain electrode metal 6 is deposited bysputtering (FIG. 1C). The source/drain electrode metal is, for example,an aluminum film or a layered film composed of aluminum and a metalhaving a high melting point, such as titanium.

[0125] Subsequent to this, a photoresist 7 is formed over thesource/drain electrode metal 6 (FIG. 1D), and an opening is formed byphotolithography and etching in a portion thereof which is above thechannel region. In this etching step, the low resistivity semiconductorfilm 5 is not etched.

[0126] Subsequently, the photoresist 7 is removed, and using the sourceelectrode and the drain electrode which have been formed intopredetermined shapes as a mask, the low resistivity semiconductor film 5is etched with the use of non-ionic hydrogen radicals. In this step ofetching the low resistivity semiconductor film 5, an etching apparatusof FIG. 2 is used. A back channel is thus formed (FIG. 1F). This step islater described in further detail.

[0127] Following the formation of the channel, using the apparatus ofFIG. 2, a passivation film 8 formed of a silicon nitride film is formedover the substrate surface by CVD so as to have a film thickness of 300nm. Thereafter, as shown in FIG. 1G, in a predetermined portion thereof,an opening is formed by photolithography and etching. This opening 9 isused for connecting a pixel electrode therethrough. Thus, a bottom gateTFT of Example 1 is completed. A schematic plan view of the TFT thusfabricated is illustrated in FIG. 3. Note that FIG. 1G shows a crosssectional view taken along the line A-A′ in FIG. 3.

[0128] Now, the etching step using non-ionic hydrogen radicals, theetching step which utilizes the above-mentioned etching apparatus, isdiscussed in detail. FIG. 2 shows a schematic view of the etchingapparatus used for making the TFT of Example 1.

[0129] As shown in FIG. 2, first, a substrate 23 which has beensubjected to the steps shown in FIGS. 1A to 1E is placed on a substrateheater 26, which also serves as a table. Thereafter, a vacuum chamber 21is evacuated using a vacuum pump. Thereafter, by the substrate heater26, the substrate 23 is heated to 250° C., for example, andsimultaneously, electricity is supplied from an external power supply 25to a metal wire 22 composed of a tungsten wire having a diameter ofabout 0.2 to 0.8 mmΦ so as to heat the metal wire 22 to about 1400° C.to about 2100° C. In this state, hydrogen, for example, is introducedfrom a gas inlet 24 into the vacuum chamber 21 to produce a hydrogen gasatmosphere at a relatively low pressure, at about 10 Pa. Thereby, thehydrogen gas in the vacuum chamber 21 makes contact with the metal wire22 and causes a contact-decomposition reaction, thereby producingnon-ionic hydrogen radicals. The hydrogen radicals collide with thesubstrate 23 due to their molecular motion and thereby etch the portionof the low resistivity semiconductor film which is not covered by thesource/drain electrode 6 (FIG. 1F). Thereby a back channel is formed.

[0130] The etching rate in the above-described etching step can bevaried by varying such conditions as temperature of the metal heatingwire, substrate temperature, gas pressure in the chamber, distancebetween the metal heating wire and the substrate, and so forth. Byproperly setting these conditions, a high quality channel which sufferslittle from etching damage is formed at high efficiency. The presentinventors have confirmed that even when the etching rate is 200 nm/minor greater, high quality TFTs are fabricated with uniform TFTcharacteristics.

[0131] It has been described that hydrogen is used for the source ofexcited species in the above-described channel etch step, but the sourceof excited species (molecules of a chemical substance) is not limited tohydrogen. Other examples usable therefor include ammonia, N₂H₄, watervapor, and a gas mixture of these elements.

[0132] This example has illustrated a case wherein hydrogen radicals areused for the channel etch, but it is to be understood that the presentinvention is not limited thereto. For example, halogen radicals such asfluorine radicals may be employed in place of hydrogen radicals. For thesource of fluorine radicals (molecules of a chemical substance), NF₃ andCF₄ may be used, for example.

[0133] The excited species used in the channel etch step may not beradicals insofar as the excited species are non-ionic excited species.Non-ionic excited species are not accelerated by a self-bias and,consequently, little etching damage is caused.

[0134] If the back channel formed by the etching as described above isexposed to atmosphere, the surface thereof is contaminated and therebythe characteristics of the TFT are degraded. In view of this, it ispreferable that the deposition of the passivation film 8 be successivelycarried cut immediately after the formation of the back channel. Inconsideration of this, the above-described etching apparatus can also beutilized as a catalytic CVD apparatus, making it possible tosuccessively carry out the formation of the passivation film 8immediately after the formation of the back channel. Thus, by the methodof the present Example, a bottom gate TFT is achieved in whichcontamination of the back channel surface is prevented.

[0135] The excited species such as hydrogen radicals can be generated byproducing radio-frequency plasma in a hydrogen gas atmosphere at apressure of about 100 Pa, for example. However, this method producesionic excited species as well. The ionic excited species are undesirablein that they are accelerated by the direct-current component generatedby radio-frequency plasma (i.e., a self-bias) and consequently causeso-called etching damage to the high resistivity semiconductor film,which lies below the low resistivity semiconductor film. The excitedspecies generated by the above-described etching apparatus are, on thecontrary, non-ionic excited species. Furthermore, due to the fact thatthe metal heating wire 22 is used for generating excited species, littleself-bias is generated. Therefore, etching damage is remarkably reduced.Accordingly, it is preferable to use excited species generated bycontacting gaseous molecules of, for example, hydrogen gas (suchmolecules are referred to as “molecules of a chemical substance” herein)with a metal wire heated by electric resistance heating to decompose themolecules.

[0136] It is to be understood, however, that, in the present invention,the excited species generated by radio frequency plasma may also beused. If this is the case, it is necessary to employ a so-called remoteplasma system. In the remote plasma system, an apparatus other than theabove-described etching apparatus, for example, a microwave plasmaapparatus or the like, is used to generate excited species. Then, amongthe generated excited species, only the non-ionic excited species areselected by, for example, guiding the generated excited species througha mesh electrode to which a bias is applied, and then the non-ionicexcited species are guided into the above-described etching apparatusfor an etching step. FIG. 5 shows an apparatus for such a remote plasmasystem.

[0137] Illustrated in FIG. 5 are a radio frequency plasma generatingchamber 56 and a gas inlet 54 also serving as an upper electrode, inwhich a mesh electrode for removing ionic excited species isincorporated. FIG. 5 also shows a chamber 51, a substrate heater 53 alsoserving as a lower electrode, and a high frequency electrode 55.

EXAMPLE 2

[0138] Example 2 shows a method of producing TFTs that includes, inplace of the channel etch step in Example 1, a step of nitriding aportion (nitrided region) of the low resistivity semiconductor film thatis located immediately above the portion of the high resistivitysemiconductor film that later forms the channel with the use of nitrideradicals or active NH_(x), where x is in the range of 1 to 3.

[0139]FIGS. 4A to 4G show cross sectional views each of which shows astep of the producing method of Example 2. The steps shown in FIGS. 4Athrough 4E are similar to those in the foregoing Example 1 above andtherefore not further elaborated upon. The steps to be carried outsubsequent to the step of FIG. 4E are described below.

[0140] The channel etch step of Example 2 also uses the apparatus ofFIG. 2, and the procedure is basically the same as that in the foregoingExample 1. Specifically, first, a substrate which has been subjected tothe steps shown in FIGS. 4A to 4E is placed on the substrate heater 26,which also serves as a table. Thereafter, the vacuum chamber 21 isevacuated using a vacuum pump. Thereafter, by the substrate heater 26,the substrate 23 is heated to 300° C., for example, and simultaneously,electricity is supplied from an external power supply 25 to the metalwire 22 composed of a tungsten wire having a diameter of about 0.2 to0.8 mmΦ so as to heat the metal wire 22 to about 1000° C. to about 1800°C. In this state, ammonia is introduced from a gas inlet 24 into thevacuum chamber 21 to produce an ammonia gas atmosphere at a relativelylow pressure, at about 10 Pa. Thereby, the ammonia gas (i.e., moleculesof a chemical substance) in the vacuum chamber 21 makes contact with themetal wire 22 and causes a contact-decomposition reaction, therebyproducing a non-ionic decomposed product (nitrogen radicals or NH_(x),where x is in the range of 1 to 2). The non-ionic decomposed productcollides with the substrate 23 and thereby nitrides the portion of thelow resistivity semiconductor film which is not covered by thesource/drain electrode 6 (FIG. 4F). Thereby a nitride film (alsoreferred to as “nitrided region” herein) is formed, and a channel isformed therebelow.

[0141] Like Example 1, few ionic excited species are generated in thismethod, and therefore, according to the above-described method, thechannel is free from the damage that is caused by the excited speciesbeing accelerated by the self-bias. Furthermore, the channel isprotected by the nitrided film 48 so as not to be in contact withatmosphere. Accordingly, highly reliable TFTs are produced with littlevariation in TFT characteristics.

[0142] The substrate temperature is set at about 300° C. because atabout 300° C., nitriding proceeds smoothly. The temperature of the metalwire 22 is set in the range of from about 1000° C. to about 1800° C.because this temperature range is preferable in that the etching that iscaused by the hydrogen radicals generated by ammonia (NH₃) does noteasily occur, and the nitriding occurs predominantly.

[0143] It has been described that ammonia is used for the source of thenon-ionic decomposed product. However, the source of the non-ionicdecomposed product is not limited to ammonia but various other chemicalcompounds may be used insofar as nitrogen atoms are contained therein.It is preferable that molecules of a chemical substance be easilydecomposed by contacting. An example of such a chemical compound isN₂H₄.

[0144] The rate of nitriding can be adjusted by varying such conditionsas temperature of the metal wire, substrate temperature, gas pressure,and distance between the metal wire and the substrate.

[0145] In FIG. 2, the metal wire 22 is shown as an example of the metalfor the contact-decomposition reaction, but the shape of the catalyticmetal is not limited thereto. Plate-like shapes and coil-like shapes maybe employed (this also applies to the foregoing Example 1).

[0146] Illustrated in FIGS. 4A to 4G are a substrate 41, a gateelectrode metal 42, a gate insulating film 43, a high resistivitysemiconductor thin film 44, a low resistivity semiconductor thin film45, source/drain electrodes 46, a resist 47, a nitrided region 48, apassivation film 49, and an opening 410.

[0147] The Relationship Between the Catalytic Metal and TFTCharacteristics

[0148] It has been described in the foregoing Examples 1 and 2 thattungsten is used as a catalytic metal in the contact-decompositionreaction, but the catalytic metal usable in the present invention is notlimited to tungsten. The present inventors have confirmed in experimentsthat similar effects are obtained when the catalytic metal is tungsten,molybdenum, tantalum, vanadium, platinum, thorium, or an alloy made fromat least two of these metals. In the experiments, it has been found thatwhen the contact-decomposition reaction which uses a metal heated byelectric resistance heating (catalytic metal) is applied to a channeletch, the catalytic metal contaminates the channel, though the amount ofthe contaminant is very little. It is thought that even if the heatingis carried out at a temperature lower than the melting point,evaporation of the metal occurs and the evaporated metal contaminatesthe semiconductor layer. In view of this problem, the present inventorshave carried out further research and as a result found the following.

[0149] (1) The amount of the catalytic metal that contaminates the TFTchannel greatly varies, depending on various conditions such as types ofcatalytic metals, temperature of heating, processing time for thechannel etch, density of the oxygen remaining in the chamber, and soforth.

[0150] (2) When conditions are the same, the use of tungsten as acatalytic metal can reduce the amount of the metal that contaminates thechannel. The reason is believed to be that tungsten has the highestmelting point among all the metals and a low vapor pressure.

[0151] (3) The amount of the catalytic metal that contaminated thechannel was an atomic density of 10¹⁶·cm⁻³ under the followingconditions: tungsten was used as the catalytic metal, the temperature ofthe metal was 1800° C., the processing time for the channel etch wasabout 1 minute, the partial pressure of hydrogen was 10 Pa, and thepartial pressure of oxygen in the chamber (the amount of remainingoxygen) was sufficiently low (about 10⁻⁴ Pa or lower). When the partialpressure of oxygen in the chamber (the amount of remaining oxygen) washigher (about 10⁻² Pa) and the rest of the conditions are the same, theamount was an atomic density of 10¹⁹·cm⁻³. Thus, the density of theremaining oxygen in the chamber greatly affects the amount of thecontamination, and the reason is believed to be that the melting pointof metal decreases when the metal becomes an oxide and therefore, theoxidized metal tends to be more easily evaporated.

[0152] (4) It has also been found that even when the density of theremaining oxygen is low, an increase in temperature of the catalyticmetal increases the amount of the contaminant. For example, when thetemperature of the catalytic metal was 2100° C. and the processing timewas 1 minute, the amount of tungsten, the contaminant, was an atomicdensity of 10¹⁹·cm⁻³. However, it has been confirmed that the TFT inwhich tungsten is contained at an atomic density of 10¹⁹·cm⁻³ in thechannel made of a silicon thin film sufficiently functions as atransistor Tungsten is a particularly preferable catalytic metal for theelectric resistance heating because it does not form a deep level insilicon, and therefore, the probability of carrier disappearance doesnot greatly vary.

EXAMPLE 3

[0153] The TFTs made in accordance with the foregoing Example 1 areformed on an insulating substrate so that they are arranged in a matrixconfiguration. Pixel electrodes are then formed to be connected to therespective drain electrodes, and using the TFTs, a liquid crystaldisplay device was produced in a known manner. It was confirmed thatwhen video signal was inputted to the liquid crystal display device tooperate the TFTs, the switching operation of each of the pixels wasaccurate in comparison with cases where TFTs made in accordance with theprior art methods, and that as a consequence, good display images wereobtained.

EXAMPLE 4

[0154] In a similar manner to the foregoing Example 3, a liquid crystaldisplay device was produced using the TFTs made in accordance withExample 2. Video signal was inputted into the liquid crystal displaydevice thus produced to operate the TFTs. As a result, it was confirmedthat stable switching operation was obtained.

EXAMPLE 5

[0155] Organic electroluminescent display devices were produced usingthe TFTs made in accordance with the foregoing Examples 1 and 2. As inthe cases of the liquid crystal display devices, it was found thatstable display performance was obtained in comparison with cases of thedevices using the TFTs made in accordance with the prior art methods.

[0156]FIG. 6 shows a basic structure of the display devices of Examples3 to 5. Illustrated in FIG. 6 are an array substrate 71, a countersubstrate 72, gate electrode lines 73, source electrode lines 74, thinfilm transistors 75, and a black matrix 76. The reference numeral 77represents a liquid crystal material or an organic electroluminescentmaterial.

[0157] As has been described thus far, the bottom gate TFTs and themethods of producing the TFTs according to the present invention achievea remarkable reduction of etching damage in the channel etch step. Inaddition, back channel is prevented from the contamination that iscaused by the exposure of the back channel to atmosphere, leading to animprovement in TFT characteristics and consequently an increase in theproduction yield. Moreover, due to the improvement of TFTcharacteristics, a high resolution display device with a large screensize is realized. Furthermore, the reductions of the contamination andof the damage to the back channel lead to further advantages. That is,even when the thickness of the a—Si:H film is reduced, the TFTcharacteristics do not degrade. As a consequence, further advantageouseffects are obtained such as a reduction in the deposition time for thea—Si:H film and an extension of a cleaning cycle of the CVD chamber,which results in a reduction in the production tact time.

[0158] Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to be notedthat various changes and modifications will be apparent to those skilledin the art. Therefore, unless such changes and modifications depart fromthe spirit and scope of the present invention defined by the appendedclaims, they should be construed as being included therein.

What is claimed is;
 1. A method of producing a bottom gate thin filmtransistor, comprising the steps of: forming a gate electrode on aninsulating substrate; forming a gate insulating film over the gateelectrode; forming a first semiconductor thin film for a channel overthe gate insulating film; forming a second semiconductor thin film for asource and a drain over the first semiconductor thin film; processingstacked layers of the first semiconductor thin film and the secondsemiconductor thin film so as to be formed into an island; subsequent tothe step of processing stacked layers, depositing a source/drainelectrode metal over the stacked layers of the first semiconductor thinfilm and the second semiconductor thin film; etching a region of thedeposited source/drain electrode metal, the region being located abovethe channel, in the depth direction to expose the second semiconductorthin film, whereby a source electrode and a drain electrode are formed;and etching away the exposed portion of the second semiconductor thinfilm in the depth direction with the use of a non-ionic excited speciesto form a channel.
 2. The method according to claim 1, wherein thenon-ionic excited species is generated by bringing molecules of achemical substance into contact with a metal heated by electricresistance heating to decompose the molecules of the chemical substance.3. The method according to claim 2, wherein the non-ionic excitedspecies is a radical.
 4. The method according to claim 3, wherein themetal is selected from the group consisting of tungsten, tantalum,molybdenum, vanadium, platinum, and thorium, or is an alloy comprisingat least two metals selected from the group consisting of tungsten,tantalum, molybdenum, vanadium, platinum, and thorium.
 5. The methodaccording to claim 4, further comprising, subsequent to the step ofetching away the exposed portion of the second semiconductor thin film,forming a passivation film comprising a silicon nitride film in such amanner that the etched surface is not exposed to atmosphere.
 6. Themethod according to claim 5, wherein the first semiconductor thin filmis a thin film comprising silicon; and the second semiconductor thinfilm is a thin film comprising silicon and an n-type impurity.
 7. Themethod according to claim 6, wherein the silicon comprises amorphoussilicon or polycrystalline silicon.
 8. The method according to claim 1,wherein the non-ionic excited species is a non-Ionic radical.
 9. Themethod according to claim 1, further comprising, subsequent to the stepof etching away the exposed portion of the second semiconductor thinfilm, forming a passivation film composed of a silicon nitride film insuch a manner that the etched surface is not exposed to atmosphere. 10.The method according to claim 2, wherein the metal is selected from thegroup consisting of tungsten, tantalum, molybdenum, vanadium, platinum,and thorium, or is an alloy comprising at least two metals selected fromthe group consisting of tungsten, tantalum, molybdenum, vanadium,platinum, and thorium.
 11. The method according to claim 3, wherein themolecules of the chemical substance comprise hydrogen, ammonia, or amixture thereof.
 12. The method according to claim 3, wherein thenon-ionic radical is a hydrogen radical.
 13. The method according toclaim 3, wherein the non-ionic radical is a halogen radical.
 14. Themethod according to claim 13, wherein the non-ionic halogen radical is afluorine radical.
 15. The method according to claim 1, wherein the stepof etching is such that the excited species is generated in a microwaveplasma generating chamber provided in isolation from an etching chamberin which the etching is performed, and from the generated excitedspecies, only non-ionic excited species are selected and introduced intothe etching chamber.
 16. The method according to claim 15, wherein theselected non-ionic excited species is a non-ionic radical.
 17. Themethod according to claim 15, further comprising, subsequent to the stepof etching away the exposed portion of the second semiconductor thinfilm, forming a passivation film comprising a silicon nitride film insuch a manner that the etched surface is not exposed to atmosphere. 18.A method of producing a bottom gate thin film transistor, comprising thesteps of: forming a gate electrode on an insulating substrate; forming agate insulating film over the gate electrode; forming a firstsemiconductor thin film for a channel over the gate insulating film;forming a second semiconductor thin film for a source and a drain overthe first semiconductor thin film; processing stacked layers of thefirst semiconductor thin film and the second semiconductor thin filminto an island; subsequent to the step of processing stacked layers,depositing a source/drain electrode metal over the stacked layers;etching a region of the deposited source/drain electrode metal, theregion being located above the channel, in the depth direction to exposethe second semiconductor thin film, whereby a source electrode and adrain electrode are formed; and nitriding the exposed portion of thesecond semiconductor thin film using a non-ionic nitrogen-containingdecomposition product that is produced by decomposing molecules of achemical substance containing nitrogen atoms.
 19. The method accordingto claim 18, wherein the non-ionic nitrogen-containing decompositionproduct is generated by bringing a metal heated by electric resistanceheating into contact with the molecules of the chemical substancecontaining nitrogen atoms.
 20. The method according to claim 19, whereinthe molecules of the chemical substance comprise ammonia.
 21. The methodaccording to claim 19, wherein the metal is selected from the groupconsisting of tungsten, tantalum, molybdenum, vanadium, platinum, andthorium, or is an alloy comprising at least two metals selected from thegroup consisting of tungsten, tantalum, molybdenum, vanadium, platinum,and thorium.
 22. The method according to claim 19, wherein the firstsemiconductor thin film is a thin film comprising silicon; and thesecond semiconductor thin film is a thin film comprising silicon and ann-type impurity.
 23. The method according to claim 22, wherein thesilicon comprises amorphous silicon or polycrystalline silicon.
 24. Abottom gate thin film transistor comprising: a gate electrode formed onan insulating substrate; a gate insulating film formed over the gateelectrode; a channel comprising a first semiconductor thin film stackedover the gate insulating film; a source region and a drain region eachcomprising a second semiconductor thin film that is stacked over aregion of the first semiconductor thin film exclusive of the channel; asource electrode and a drain electrode formed on the secondsemiconductor thin film; and a passivation film composed of a siliconnitride film formed on the channel; wherein a portion of the channelcontains at least one element selected from the group consisting oftungsten, tantalum, molybdenum, vanadium, platinum, and thorium, theportion of the channel being adjacent to a surface thereof which is incontact with the silicon nitride film; and the total atomic density ofthe at least one element is in the range of from 1×10¹⁶·cm⁻³ to1×10¹⁹·cm⁻³.
 25. The bottom gate thin film transistor according to claim24, wherein the first semiconductor thin film is a thin film comprisingsilicon; and the second semiconductor thin film is a thin filmcomprising silicon and an n-type impurity.
 26. The bottom gate thin filmtransistor according to claim 25, wherein the silicon comprisesamorphous silicon or polycrystalline silicon.
 27. A bottom gate thinfilm transistor comprising: a gate electrode formed on an insulatingsubstrate; a gate insulating film formed over the gate electrode; achannel formed of a first semiconductor thin film stacked over the gateinsulating film; a source and a drain each formed of a secondsemiconductor thin film stacked over the first semiconductor thin film;a nitrided region in which a portion of the second semiconductor thinfilm disposed immediately above the channel is nitrided; and a sourceelectrode and a drain electrode, each formed on a portion of the secondsemiconductor thin film exclusive of the nitrided region; wherein aportion of the channel contains at least one element selected from thegroup consisting of tungsten, tantalum, molybdenum, vanadium, platinum,and thorium, the portion of the channel being adjacent to a surfacethereof which faces the nitrided region; and the total atomic density ofthe at least one element is in the range of from 1×10¹⁶·cm⁻³ to1×10¹⁹·cm⁻³.
 28. The bottom gate thin film transistor according to claim27, wherein the first semiconductor thin film is a thin film comprisingsilicon; and the second semiconductor thin film is a thin filmcomprising silicon and an n-type impurity.
 29. The bottom gate thin filmtransistor according to claim 28, wherein the silicon comprisesamorphous silicon or polycrystalline silicon.
 30. A liquid crystaldisplay device comprising: a first substrate comprising a plurality ofscan electrodes, a plurality of data electrodes intersecting the scanelectrodes, a plurality of thin film transistors provided at theintersectional positions of the scan electrodes and the data electrodesso that at least one of the plurality of thin film transistors isprovided at each of the intersectional positions, and a plurality ofpixel electrodes connected to the thin film transistors; a secondsubstrate comprising a counter electrode opposed to the pixelelectrodes; and a liquid crystal sandwiched between the first substrateand the second substrate; wherein each of the thin film transistors is abottom gate thin film transistor according to any one of claims 24 to29.
 31. An organic electroluminescent display device comprising: a firstsubstrate comprising a plurality of scan electrodes, a plurality of dataelectrodes intersecting the scan electrodes, a plurality of thin filmtransistors provided at the intersectional positions of the scanelectrodes and the data electrodes so that at least one of the thin filmtransistors is provided at each of the intersectional positions, and aplurality of pixel electrodes connected to the thin film transistors; asecond substrate comprising a counter electrode opposed to the pixelelectrodes; and a layer comprising an organic electroluminescentmaterial, the layer being sandwiched between the first substrate and thesecond substrate; wherein each of the thin film transistors is a bottomgate thin film transistor according to any one of claims 24 to 29.